Analytic philosophers are often accused of not taking existential questions seriously. This may be true of some (many?) of them, but it is certainly not true of the contemporary Oxford philosopher Derek Parfit. He has revived an interest in existential questions within analytic philosophy. He has discussed the nature of personal identity, why there is something rather than nothing and, in particular, questions to do with our own existence, and the existence of creatures like us. From the point of view of morality,

Is it a good thing that I exist?

Is it a good thing that other creatures like me exist?

Is it better if there are more creatures like me rather than less?

Is it possible to make the world a better place, not only by making existing creatures happier, but also by creating additional happy creatures?

Obviously, the answers to these questions have a clear and direct bearing on our own lives, on our most intimate reproductive decisions. These questions used to be neglected in philosophy, but Derek Parfit has placed them at the centre of the discussion.

It is well known, at least among professional philosophers, that classical utilitarianism has the implication that there are two ways whereby we can make the world a better place:

by making existing creatures happier, or

by making happy additional creatures.

This means that the question of whether, from the point of morality, it is a good thing that you and I exist, receives a positive answer (to the extent that we lead lives worth experiencing, and to the extent that we do not cause harm to others). It also means that we may have a positive obligation to procreate and replenish the world. However, utilitarianism, like many similar (total) views, leads to what Parfit has called the Repugnant Conclusion. This is the conclusion that, for any possible population of at least ten billion people, all with a very high quality of life, there must be some much larger imaginable population whose existence, if other things are equal, would be better, even though its members have lives that are barely worth living.

We arrive at this conclusion if we accept that a loss in quality could always be outweighed by a sufficient gain in the quantity of well-being. With this view, a more numerous population, where each individual is living a worse life (but still a life worth living), is better than a given population living at a high level of well-being, if it is large enough. If we call the first population described by Parfit the A-population, and the second (vast) population the Z-population, we can imagine a moral alphabet, where we move from the A-population via intermediate populations, much more numerous, but with a lower level of well-being, all the way down the moral alphabet to the Z-population. A way of illustrating this seemingly absurd exercise in moral mathematics is the following: [Diagram omitted] Parfit is not the first philosopher who has noticed that many influential moral views lead to the Repugnant Conclusion. Henry Sidgwick was close to acknowledging this implication when he pointed out that "the point up to which, on utilitarian principles, population ought to be encouraged to increase, is not that at which the average happiness is the greatest possible - as appears to be often assumed by political economists of the school of Malthus - but that at which the happiness reaches its maximum" (1907 p. 418). Corresponding observations have been made by other philosophers. However, it is Parfit who has brought the conclusion to recent philosophical attention, and he has done so, both by stressing the importance of the conclusion as such, and by showing how difficult it is to avoid it. And even though this is the first anthology devoted exclusively to the theme, the number of scholarly articles dealing with the Repugnant Conclusion in recent discussion is legion. One need only think for a short while about the subject to understand why this is so.

Most people (including moral philosophers), when faced with the fact that some of their cherished moral views lead up to the Repugnant Conclusion, feel that they have to revise their moral outlook. However, it is a moot question as to how this should be done. It is not an easy thing to say how one should avoid the Repugnant Conclusion, without having to face even more serious implications from one's basic moral outlook. Several such attempts are presented in this volume.

One idea put forward by Derek Parfit in his contribution is that there is discontinuity between values, in the sense that a loss of the things which do most to make life worth living cannot be made good by any gain in the quantity of more inferior values. This means that the move from the A-world to the Z-world is brought to a halt. In fact, Parfit suggests that discontinuity sets in at the very beginning: the move is blocked in the first step from world A to world B.

A view with some affinity to Parfit's proposal is presented by Tim Mulgan in his critical-level theory: "Mulgan (Tim) - Two Parfit Puzzles". Mulgan draws on a Kantian or Rawlsian idea about a minimal threshold of liberties and primary goods, where it is imperative for society to ensure that everyone lives above that threshold. Such a threshold should be taken seriously, he claims, but we must acknowledge that it is context-dependent. In particular, the more affluent a society becomes, the higher the critical-level will be pushed.

Another twist to the critical-level view is given by Charles Blackorby, Walter Bossert and David Donaldson in their joint contribution to the volume: "Blackorby (Charles), Bossert (Walter) & Donaldson (David) - Critical-Level Population Principles And The Repugnant Conclusion". Their formal approach allows them to demonstrate that no critical-level generalized-utilitarian principle can avoid both the Repugnant Conclusion and the strong 'sadistic' conclusion, according to which, for any alternative in which everyone's utility is negative, there exists a worse alternative in which everyone's utility is positive. The strong sadistic conclusion may seem as hard, or harder, to digest than the Repugnant Conclusion, but the authors argue that it is acceptable. The very fact that the critical-level view means a radical split between prudential and moral value may be seen as problematic. And, as has been demonstrated for example in Blackorby et al, there are other difficulties with the critical-level view as well. So some philosophers have taken up other tacks.

Another theory which manages to block the Repugnant Conclusion is presented by Clark Wolf in "Wolf (Clark) - O Repugnance, Where Is Thy Sting?". Inspired by Karl Popper's contention that the utilitarian formula "Maximize happiness" should be replaced by the formula "Minimize suffering". Wolf advocates an ethical theory according to which it is praiseworthy but not obligatory to promote well-being, but obligatory to reduce ill-being. This theory does not entail that world Z is preferable to world A. However, neither does it entail that world A is preferable to Z. Whether the latter implication is acceptable is discussed by bringing in considerations on what it is like to lead a life in world Z. Wolf's discussion, thereby, anticipates a question which constitutes the turning point in the final contributions to the volume.

In "Parfit (Derek) - Reasons and Persons" Parfit conjectured that one way of avoiding the Repugnant Conclusion would be to take the value of the quantity of well-being to be in one way or the other 'bounded'. The more sentient creatures that exist, the less important it becomes to add to their number. This idea is more flexible than the 'lexical' critical-level view, and it is taken up and defended in this volume by Tyler Cowen in "Cowen (Tyler) - Resolving The Repugnant Conclusion". A standard objection to this move has been, he observes, that it is ad hoc. Cowen, building on a comparison with Pascal's Wager, attempts to show that the common objections to bounding do not succeed, however, if the bound is defined properly.

Another move discussed by Parfit would be to introduce some kind of person-affection restriction to our moral reasoning. Not all sorts of merely hypothetical individuals count in our moral mathematics. There are different ways of conceiving of this restriction. Some have wanted to restrict attention to 'actual' creatures (those who, as a matter of fact, have lived, do live, or will come to live), to 'necessary' creatures (those who will come to live no matter what the agent does), or to 'present' individuals. The person-affecting restriction is at the centre of two contributions to this volume.

Nils Holtug, in contrast, in "Holtug (Nils) - Person-Affecting Moralities", discusses these various different possibilities and finds fault with all of them. The only (morally) plausible interpretation of the person-affection restriction, he claims, is an interpretation which does not succeed in avoiding the Repugnant Conclusion.

No matter whether one believes that morality should take a person-affecting or an impersonal form, it is a fact that the discussion about the Repugnant Conclusion has typically taken place from a teleological point of departure. This means that we assess a value to the consequences of our actions. An action is right if it maximises value. Accepting this framework, could we not give up the idea that "better than" is a transitive relation? This, it would seem, would allow us to disarm all sorts of arguments pushing us, stepwise, down the moral alphabet. This idea is taken up by Ingmar Persson in his contribution to the volume ("Persson (Ingmar) - The Root of The Repugnant Conclusion And Its Rebuttal"). Moreover, it is also considered by Gustaf Arrhenius and, in a somewhat different context, by Stuart Rachels. While Persson defends the abandonment of transitivity, Arrhenius – in "Arrhenius (Gustaf) - The Paradoxes Of Future Generations And Normative Theory" – is much more sceptical. In fact, his purpose is to defend an impossibility theorem which implies that we cannot exorcise the paradoxes of population ethics either by giving up the transitivity of "better than" or by rejecting consequentialism and switching to a non-consequentialist framework. Rachels, for his part, does not wish to take sides in the dispute on transitivity. Rather his purpose – in "Rachels (Stuart) - Repugnance or Intransitivity: A Repugnant But Forced Choice" – is to show that population ethics is faced with a dilemma: either we must give up transitivity or else accept the Repugnant Conclusion (or some variant of the conclusion).

As we saw, Nils Holtug argued that the person-affecting restriction cannot block the Repugnant Conclusion. Having conceded this, he goes on to bite the bullet and to accept the Repugnant Conclusion. The same move has been made by the two editors of this volume in their respective contributions.

Torbjörn Tännsjö argues, first of all in "Tännsjö (Torbjörn) - Why We Ought To Accept The Repugnant Conclusion", with reference to a claim made by the late John Mackie, that our moral 'intuition' that the A-world is better than the Z-world, is not reliable. In particular, Tännsjö claims, there is something wrong with the idea of people leading lives way above the level where life becomes worth living. We know little about what it is like to live such a life, he contends somewhat pessimistically. When this putative fact is contemplated, the repugnant conclusion does not strike one as obviously wrong, he claims. And since there are good (even compelling) reasons to accept the Repugnant Conclusion, this is what we ought to do.

Jesper Ryberg, finally, reaches a similar conclusion – in "Ryberg (Jesper) - The Repugnant Conclusion and Worthwhile Living" – on the ground of the pessimist premise that a life in the Z-world does not, at a daily level, differ much from a normal privileged life. His purpose is to defend this premise by providing explanations as to why the typical view on the matter may be deluded and, furthermore, to defend the contention that the pessimist premise is crucial with regard to the repugnance of the Repugnant Conclusion.